This invention relates to intruder detection systems and, more particularly, to improvements in signal processing for the purpose of minimizing any tendency for false alarms.
In U.S. Pat. No. 4,612,442 issued to Toshimichi, there is disclosed a passive infrared intrusion detection system comprising circuitry for processing digital signals to minimize the effects of spurious false-alarm-producing sources. Such circuitry includes a pulse width discriminator for eliminating so-called "popcorn" noise, and a digital counter for counting potential alarm-producing pulses produced by the infrared radiation-sensitive detector element of the system. Only in the event that a predetermined count is reached within a certain time interval (determined by a timing circuit) is an alarm relay activated.
In the above-mentioned intruder detection system, the time interval during which pulses are counted is initiated by the first pulse transmitted by the pulse discriminator. Once initiated, the time interval times out for the selected time period (usually about 20-30 seconds). If the requisite number of pulses is not counted during that period, no alarm is sounded, and the pulse that initiated the time interval, as well as those counted pulses which are less than the number required for alarm activation, are assumed to have been produced by something other than an intruder.
In order to assure that the above system will detect intruders at long range, the time interval must be sufficiently long as to allow a slow moving intruder to cross two target fields (i.e., two fields of view of the detector element). Obviously, if the time interval is set for a relatively long period, say, several minutes, spurious signals spaced minutes apart (not unusual) can produce false alarms. On the other hand, if the time interval is set relatively short, say, for only a few seconds, a slow moving intruder can go undetected. With these two consideratons in mind, a time interval of between 20 and 30 seconds is usually selected.
While the digital signal processing circuitry described in the above-mentioned Toshimichi patent may be effective in discriminating many false-alarm-producing events from those attributable to intrusion, such circuitry is nevertheless susceptible to certain types of spurious sources. For example, in the case of a passive infrared system of the type having extremely sensitive pyroelectric sensors, if a heater is turned on in the region under surveillance, the sensors can saturate, producing a first pulse at the outset of such event, and second and third pulses, perhaps 20 seconds later as the sensors come out of saturation and settle to a steady-state condition. Assuming the system is set to alarm after counting 3 pulses within a 25 second time window, such an event would give rise to a false alarm. Thus, it would be very desirable to shorten the time interval during which pulses are counted without sacrificing the "catch" performance at long range. Also, it would be desirable to reduce the cost of signal-processing circuitry of the above system (which requires a relatively costly digital counter) without sacrificing the effectiveness of such systems in minimizing false alarms.